1. Field of the Invention
The present invention relates generally to the packaging of semiconductor sensors, actuators, and devices, and specifically, to packaging of semiconductor pressure sensors.
2. Background Information
Conventional semiconductor devices are packaged using a variety of methods including low cost, injection molding of plastic around the semiconductor die and leadframe. Certain types of semiconductor devices such as pressure sensors, actuators, and stress-sensitive electronic circuits typically cannot be injection molded in the conventional approach without degrading or impairing device operation. Semiconductor pressure sensors based on deformable diaphragms, for example, will typically shift in output signal, sensitivity, non-linearity, and hysteresis if not adequately isolated from contact with rigid materials.
Due to high stress sensitivity of the sensors, one method involves attaching a sensor die to a back plate having a closely matched thermal coefficient of expansion (e.g., Pyrex, silicon, or glass), which is then attached to a substrate. Typically, the substrate used is ceramic, PCB, plastic, or a metal header. Depending on the application and degree of accuracy required, the sensor die is attached to the substrate with soft epoxies or RTVs. The soft die attach material acts as a stress isolation mechanism, dampening the transfer of external stresses to the sensor die. The sensor package is housed in a protective enclosure in an effort to eliminate any physical contact with the sensor die itself (other than the measured media). This has typically resulted in a more complicated packaging and manufacturing process for the sensor. Although silicon sensors are similar to integrated circuits, sensor manufacturers have not been as successful, from a cost and reliability standpoint, in utilizing the advances in semiconductor packaging and manufacturing.
FIG. 1 illustrates a cross-sectional view of a prior-art semiconductor pressure sensor 1 housed in a plastic package 3. The pressure sensor 1 includes a semiconductor sensor die 2 having a thin, pressure-sensitive diaphragm 9 that is bonded to the bottom of the plastic package 3 using silicone RTV, as shown by numeral 4. The sensor die 2 typically contains a series of stress-sensitive resistors connected in a Wheatstone bridge configuration to provide a voltage output which varies with pressure applied across the diaphragm 9. Bond wires 13 are used to connect the stress-sensitive resistors to a leadframe 5, which is supported by the plastic package 3. A lid 7 is attached over the plastic package 3 and contains a hole 8 for the application of external pressure from a top side 11 of the pressure sensor 1. A second pressure port 10 may be optionally provided from a bottom side 12 of the sensor 1 for gage or differential pressure measurements.
The sensor die 2 is typically coated with a gel 6 to prevent direct environmental exposure and corrosion. The gel 6 is a compliant material, capable of transferring pressure to the diaphragm 9. Although at first, the semiconductor package seems like an elegant solution to the environmental exposure issue, the gel is not impervious to gases and liquids. That is, gases and liquids tend to penetrate the gel, corroding the underlying metal traces, bond pads, and wire bonds. Conventional integrated circuit packaging materials such as thermosetting plastics offer improved environmental protection, though they have historically been unsuitable for pressure sensors since they are rigid, non-compliant, thermally mismatched, and hysteretic, preventing cost-competitive packaging techniques to be utilized.